The term "polymer" refers herein to materials comprising large molecules built up by the repetition of small, simple chemical units. In a hydrocarbon polymer those units are predominantly formed of hydrogen and carbon. Polymers are defined by average properties, and in the context of the invention polymers have a number average molecular weight ("M.sub.n ") of at least about 500. The term "hydrocarbon" refers herein to non-polymeric compounds comprising hydrogen and carbon having uniform properties such as molecular weight. However, the term "hydrocarbon" is not intended to exclude mixtures of such compounds which individually are characterized by such uniform properties.
Hydrocarbon compounds and hydrocarbon polymers have been reacted to form carboxyl group-containing compounds and polymers and their derivatives. Carboxyl groups have the general formula --CO--OR.sup.w, where R.sup.w can be H, a hydrocarbyl group, or a substituted hydrocarbyl group. The synthesis of carboxyl group-containing compounds from olefinic hydrocarbon compounds, carbon monoxide, and water in the presence of metal carbonyls is disclosed in references such as H. Bahrmann, Chapter 5, Koch Reactions, "New Synthesis with Carbon Monoxide", J. Falbe; Springer-Verlag, New York, 1980. Hydrocarbons having olefinic double bonds react in two steps to form carboxylic acid-containing compounds. In the first step an olefin compound reacts with an acid catalyst and carbon monoxide in the absence of water. This is followed by a second step in which the intermediate formed during the first step undergoes hydrolysis or alcoholysis to form a carboxylic acid or ester. An advantage of the Koch reaction is that it can occur at moderate temperatures of -20.degree. C. to +80.degree. C., and pressures up to 100 bar.
The Koch reaction can occur at double bonds where at least one carbon atom in the double bond is di-substituted to form a "neo" acid or ester, such as can be represented by formula: ##STR1## wherein R.sup.x and R.sup.y are the same or different hydrocarbyl groups and R.sup.z is H or hydrocarbyl. The Koch reaction can also occur when both carbons are mono-substituted or one is mono-substituted and one is unsubstituted to form an "iso" acid or ester; e.g., --R.sup.y HC--COOR.sup.z.
Bahrmann et al. discloses the conversion of isobutylene to isobutyric acid via a Koch-type reaction. U.S. Pat. No. 2,831,877 discloses a multi-phase, acid catalyzed, two-step process for the carboxylation of olefins with carbon monoxide. Complexes of mineral acids in water with BF.sub.3 have been studied to carboxylate olefins. Examples of such complexes are H.sub.2 O.cndot.BF.sub.3 .cndot.H.sub.2 O, H.sub.3 PO.sub.4 .cndot.BF.sub.3 .cndot.H.sub.2 O and HF.cndot.BF.sub.3 .cndot.H.sub.2 O. U.S. Pat. No. 3,349,107 discloses processes which use less than a stoichiometric amount of acid as a catalyst.
U.S. Pat. No. 5,629,434 discloses the production of functionalized polymers containing (thio)carboxylic acid or ester groups via the Koch reaction, wherein polymers with M.sub.n of at least about 500 and having at least one ethylenic double bond are reacted with carbon monoxide in the presence of an acid catalyst and a nucleophilic trapping agent selected from water, hydroxy containing compounds and thiol containing compounds. U.S. Pat. No. '434 does not disclose the continuous production of functionalized polymers.
U.S. Pat. No. 5,650,536 discloses the continuous production of functionalized polymers via the Koch reaction using a continuous stirred tank reactor or a tubular reactor. U.S. Pat. No. '536 particularly discloses continuously reacting a starting polymer, an acid catalyst such as gaseous BF.sub.3, a nucleophilic trapping agent selected from water, alcohols, and thiols, and gaseous carbon monoxide under Koch conditions in a tubular reactor employing in-line mixers spaced at intervals along the length of the reactor to disperse the gas into the liquid and promote reaction. The intervals of open pipe between the mixers provide residence time intervals for reaction ranging from 0.25 to 5 minutes. The in-line mixers can be mechanical mixers or static mixers. U.S. Pat. No. '536 further discloses that the mixer intensity can be relaxed toward the reactor exit, since high gas-liquid contact is primarily required in the front portion of the reactor. Gas dispersing mixers are accordingly preferred in the front end of the reactor, and blending mixers in the back end. It is also disclosed that a preferred embodiment of the process includes a laminar flow process using static mixers where the Reynolds number is very low, preferably less than 10. The disclosed advantages of this reaction scheme, which can be referred to as an intermittently mixed reaction scheme, include short reaction times, high yields, no moving parts or seals (when static mixers are used), no need for liquid level control, and the production of a clean, white product especially where exposure to air and oxygen is avoided.
While suitable for use in many circumstances, the intermittently mixed reaction scheme disclosed in U.S. Pat. No. '536 for the continuous production of functionalized polymers has certain drawbacks. When static mixers provide the intermittent mixing, each mixer is a source of pressure drop, resulting in a relatively steady and significant decrease in reaction pressure from entry to exit of the reaction zone. At some point in the reaction system, the drop in pressure can lead to reduced solubility and dispersibility of the gaseous reactants and agents (e.g., CO and BF.sub.3) causing gas-liquid separation and slug flow in the open pipe between mixing zones. The gas-liquid separation results in a lower reactant concentration in the liquid phase which can reduce reaction yield. Slugging adversely affects the performance of the mixers downstream, in essence requiring the mixing/dispersing operation to start over at each intermittent mixer, thereby making less effective use of the pressure drop expended. This decreased mixer effectiveness can reduce reaction rate and yield. Pressure drop can be minimized by the use of larger static mixers and a lower fluid velocity, but this is unattractive because the shear will also be lower, which will limit the effectiveness of the mixer in achieving dispersion of the gas in the liquid.
Furthermore, the intermittently mixed reaction scheme for functionalizing polymers is generally suitable for operation only over a relatively narrow viscosity range. At viscosities above the design range, the pressure drop across the intermittent mixers and tubes can become prohibitively large, while at viscosities below the design range, the gas tends to coalesce rapidly upon exiting the mixers, thus reducing mass transfer rates. This rapid coalescence can be countered to a degree by reducing the average residence time between intermittent mixers, but this typically requires the use of additional mixers at added cost. In any event, because of these limitations, the intermittent scheme is less attractive for certain polymer functionalizations. For example, use of an intermittent reactor scheme designed for operation in a low to medium viscosity range for the functionalization of a relatively high molecular weight polymer at a relatively low reaction temperature (where a low reaction temperature is necessary to minimize side reactions) may not be desirable, because substantial dilution with an inert solvent may be required in order to maintain viscosity within the design range. The use of a large amount of diluent is disadvantageous, because it can require a substantial investment in facilities for storing and handling the diluent and for separating the diluent from the functionalized polymer product.